Plasma emission spectroscopy

ABSTRACT

In plasma emission spectroscopic analysis, of organic compounds intensities recorded for radiation characteristic of elements other than carbon is corrected for alterations in background radiation caused by carbon.

McLean et al.

[ PLASMA EMISSION SPECTROSCOPY [75] Inventors: William Ronald McLean: David Leon Stanton, both of Stockton-on-Tees, England [73] Assignee: Imperial Chemical Industries Limited, London. England [22] Filed: Oct. 23, 1973 [2]] Appl. No.: 409,004

[30] Foreign Application Priority Data Oct. 26, 1972 United Kingdom 49466/72 [52] US. Cl 356/82; 356/85 [51] Int. Cl. G0lj 3/38 [58] Field of Search 356/82, 81, 85, 86

[56] References Cited UNITED STATES PATENTS 2,577.814 l2/l95l Saunderson et a]. t. 356/81 6/l97l l0/l97] 3/l972 4/l973 [111 3,894,803 [45 July 15, 1975 Habosian 356/82 Wood 4 356/85 Habosian r l A r v. 356/82 Cooper et a]. 356/86 Primary E.\'aminerRonald J. Stern Assistant ExaminerF. L. Evans Attorney, Agent, or FirmCushman, Darby &

Cushman ABSTRACT 6 Claims, 2 Drawing Figures RE A 0 OUT AME MEANS FOFI exposure 9 saunas 1-0 A A PLASMA RADIATION W pecmomsren PHOTO- Q wunvusa 2 25' 42 Q l amiss J5 l6 MIR I2 25 44 J6 a n 5 on 2.9 9 MP. /7 22 5.1 i a t 26 45 4 FT": ,4 n r/la l Ni w 594 40 J9 f Wm:

SHEET iii? ost.

PLASMA EMISSION SPECTROSCOPY This invention relates to plasma emission spectroscopy.

In plasma emission spectroscopy we have found that the presence of carbon creates a background (continuous) radiation which increases the intensities of radiation over a broad range and thus in general increases the intensities detected at the frequencies characteristic of other species.

In plasma emission spectroscopy an elemental analysis of a carbon-containing sample may be determined by exposing the sample to a plasma, resolving radiation emitted by the plasma according to its frequency and detecting the intensity of radiation of one or more frequencies characteristic of one or more individual elements. By plasma is meant a mixture of electrons and gaseous ions optionally together with neutral atoms. The plasma may suitably be formed in the presence of neon, argon, or preferably helium, by exciting a gas, for example by subjecting it to an electric discharge, microwave radiation and/or high frequency induction. Pressures of 0.01 to 40 Torr may for example be employed using helium. The sample may be exposed to the plasma by flowing it in a gas stream through a region in which a plasma is produced and the process may be applied to the effluent recovered from gas chromatographic apparatus. Suitable techniques for performing such elemental analyses are known, such as that disclosed in copending US. Pat. application Ser. No. 135,836, now abandoned, assigned to the assignee of this invention.

This invention comprises apparatus for plasma emission spectroscopy of carbon containing samples which comprises means for exposing the sample, preferably in a gas stream, to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of carbon and of other species and means for correcting the intensity recorded for radiation of frequencies characteristic of other species to account for an increase in background radiation of that frequency due to carbon. when used for elemental analysis the sample should be substantially completely decomposed to its atoms in order to ensure that sub stantially all of each element under investigation emits radiation characteristic of the elements, i.e., atomic ra diation. If it is partly in a combined state this will reduce the intensity of the atomic radiation detected.

The means for correctingthe intensity recorded for individual elements other than carbon may, if an elemental analysis of carbon is being simultaneously determined, comprise means for deducting a value related to the carbon analysis (and optionally also related to the frequency characteristic of the individual element) from the value recorded for that element. It has been found that the background radiation due to carbon extends throughout a wide range of frequencies but that it declines in intensity towards the red end of the spectrum. It is advisable therefore to apply a different correction at a similar level of carbon analyses to frequencies characteristic of individual elements if they are widely separated and especially if any of them are at low frequencies.

The means for correcting the intensity recorded for individual elements other than carbon may alternatively comprise means for detecting radiation at a frequency different from but preferably within a range plus or minus 30A of the frequency characteristic of the individual element and deducting the signal obtained at such a frequency from that obtained at the frequency characteristic of the individual element. This means may, if the analysis is being carried out using a multi-channel spectrometer set to a frequency charactertistic of the individual element, comprise a further channel of the spectrometer set to another frequency representative of the background radiation at a frequency characteristic of the individual element and preferably within plus or minus 30A of the frequency characteristic of the individual element, and means for deducting the signal secured at this frequency from that secured at the frequency characteristic of the individual element.

According to a further form of this invention the correction may be made even if a single channel spectrometer is used. In this form of the invention the spectrometer is provided with means for securing a frequency shift in the setting of the spectrometer during the period in which the intensity of radiation of the frequency characteristic of the individual element is being secured. This means may comprise means optically to reset the spectrometer by a preset amount, preferably in a vibration-free manner. It may, therefore, comprise an element for example a sheet of a material which is refractive and transparant to the radiation, for example of quartz, interposed at an angle to the resolved radiation so as to divert radiation of a different frequency to sensing means for the radiation within the spectrometer. If desired the said means for securing a frequency shift in the setting of the spectrometer may be adjustable to vary the frequency shift. The frequency shift is conveniently achieved by interposing to transversely intersect the plane of the radiation of the frequency characteristic of the element to be analysed by rotation at least one laminar component of a material which is transparent and refractive to the radiation, the axis of rotation of the component preferably being adjustable so as to vary the angle between the plane of the radiation and the plane of rotation by a desired amount.

By transparent" is meant that the material transmits the radiation without undue scattering or diffusion effects. It will be appreciated that some radiation may be absorbed and/or reflected by the material but such losses may be allowed for, for example, by appropriate amplification or attenuation of signals derived from the spectrometer.

Any suitable mechanical formation may be employed and the means may comprise a rotatable planar disc which has holes cut in it, the disc either being of the material or having some holes occupied by the material. lt is preferred that it should serve to secure the in stantaneous shift in the frequency sensed by the spectrometer from one value to another. The effect of such a device is to secure a square wave signal from the spectrometer of which the difference between the maximum and minimum value is representative of the concentration of the element to be detected. By appropriate design of the means for producing the frequency shift a sinusoidal wave may be produced if this is preferred.

FIG. 1 is a schematic diagram of apparatus for plasma emission spectroscopy in accordance with one embodiment of the invention.

FIG. 2 is a schematic diagram of apparatus for plasma emission spectroscopy in accordance with an alternate embodiment of the invention.

One form of the invention will now be described with reference to H6. 1 which shows apparatus according to the invention in diagrammatic form. A spectrometer 1 which receives radiation from a means for exposing samples to a plasma 49 from a plasma produced in accordance with known techniques (one suitable technique being described in US. application Ser. No. l35,836, now abandoned, assigned to the assignee of this invention) by flowing a gas stream containing helium, to which stream a carbon-containing sample may be introduced, through a microwave cavity in the presence of a scavenger gas for carbon, and comprises a diffraction grating for resolving radiation from the plasma according to its frequency (not shown) and photomultipliers 2, 3, 4 and 5 positioned to detect the intensity of radiation of frequencies characteristic of four individual elements. that of photomultiplier 5 being carbon. Current passing from the photomultiplier during detection of the sample may be passed by closing switches 6, 7, 8, 9 and 14 to one plate of condensers 10, l l l2 and 13. The other plate of condensers 10, 11, 12 and 13 is brought to a potential in the opposite sense (a back-off potential) by means of differential operational amplifiers 15, 16, 17 and 18, connected in their virtual earth mode, i.e., with feed-back resistors 19, 20, 2| and 22, the back-off potential being produced by passing a potential from potentiometers 23, 24, and 26 through resistors 30, 31, 32 and 33 and also in the case of photomultipliers 2, 3 and 4 from potentiometers 27, 28 and 29 through resistors 34, 35 and 36. A potential is applied across potentiometers 23, 24, 25 and 26 with respect to earth point 37 from high impedence amplifier 38. This is adapted to pass the potential accumulated on condenser 39 as a result of passing to it a constant current from constant current source 40 during the period when switches 6, 7, 8, 9 and 14 are closed, the other plate of condenser 39 being connected to earth. Potentiometers 27, 28 and 29 are fed by means of high impedance amplifier 41 with the potential accumulated on condenser 13 with respect to earth, the other pole of the potentiometers 27, 28 and 29 being connected to earth. Sensitivity potentiometers 42, 43, 44 and 45 are connected between earth by means of a parallel switch. The adjustable tapping from potentiometers 42, 43, 44 and 45 is simultaneously connected to read-out 47.

The apparatus is operated as follows: a plasma is induced in the absence of any material to be analysed and switches 6, 7, 8, 9 and 14 are closed. A steady increase in the potential across condenser 39 results and thus a steadily increasing potential is applied across potentiometers 23, 24, 25 and 26 because of the constant flow of current to condenser 39 from constant current source 40. A proportion of this potential is applied to condensers 10, 11, 12 and 13 according to the tapping taken from the potentiometers. The other plate of condensers 10, ll, 12 and 13 is fed from photomultipliers 2, 3, 4 and 5 and main amplifier 46 and read-out 47 are connected to each photomultiplier in turn. Potentiometers 23, 24, 25 and 26 are then adjusted so that condensers 10, 11, 12, and 13 show no potential during this operation. This corrects the signal from the photomultipliers to eliminate the so-called dark current and current due to background radiation (i.e., that current which passes in the absence of sample).

A carbon-containing sample which contains no element to which photomultipliers 2, 3 and 4 are sensitive and which, therefore, excites only photomultiplier 5 is introduced to the plasma and the potential building up on condenser 13 is passed to potentiometers 27, 28 and 29. A proportion of the potential across these potentiometers passes to condensers 10, 11 and 12 according to the tapping set on these potentiometers the charge on condensers 10, 11 and 12 is scanned by means of main amplifier 46 and read-out 47. Potentiometers 27, 28 and 29 are then adjusted so as to produce a zero value of read-out during the passage of a carboncontaining sample. The result of this adjustment is that a proportion of the carbon signal is backed off against the signal for each of photomultipliers 2, 3 and 4 whatever the magnitude of the carbon signal. This compensates for the increase in background radiation at the frequencies corresponding to photomultipliers 2, 3 and 4 due to the presence of carbon in the sample. When a sample containing elements detected by photomultipliers 2, 3 and 4 is passed into the plasma a signal representative of the radiation due to that element in the absence of carbon is therefore passed to main amplifier 46.

Potentiometers 42, 43, 44 and 45 are used to adjust the sensitivity of the recorded potential characteristic of the element to a desired value at which it is desired to print it out.

A diagram of another form of the invention is shown in FIG. 2. A single channel spectrometer which is adapted to respond to a plasma as aforesaid (the means for producing the plasma and exposing samples to it identified by reference numeral 114) comprises a detection slit and a diffraction grating (not shown) which resolves radiation from the plasma by fanning it out in the plane of the drawing; a photomultiplier 101 and in the path of radiation passing to the photomultiplier a disc 110 containing four similarly shaped and sized windows of which two boundaries are radial and two are the center of curvature of the arcs about the axis of the disc 110. The disc is axially driven by motor 103. One pair of windows opposite to one another are closed by quartz plates and the other windows are empty. Means (not shown) is provided for rotating the axis of rotation of disc 110 its center remaining in the same position. The signal from photomultiplier 101 is passed to adjustable means to back off its dark current 102 and from this to phase sensitive decoder 113 which is fed by phase sensitive switch 104 coupled to disc 110. The signal passing from photomultiplier 101 when the light reaching it is passed through the empty window is passed to filter 105 which srnoothes it. The signal passing from photomultiplier 101 when the light passes through a quartz plate is passed to adjustable amplifier and inverter 106 and from this to filter 105. The signal from filter 105 is passed to means for amplifying or attenuating a signal 107 and from this to recorder 108.

The apparatus is operated as follows: Switch 109 is opened but no light is passed to the photomultiplier by closing the detection slit, and the means for dark current back-off 102 is adjusted to give a zero value on the recorder, the spectrometer set to the frequency which it is desired to detect by admitting a sample to the plasma. The plasma is then detected at the same frequency without any sample being present, switch 109 being closed, the axis of rotation of disc 110 being set to give a sufficient frequency shift of light. Amplifier and inverter 106 is then set to give a zero reading on recorder 108. This operation corrects for losses in intensity of radiation due to reflection and absorption by the quartz plates.

A sample may then be admitted to the plasma, the frequency to which the photomultiplier is tuned is shifted whenever the light passes through the quartz plate and the value of the intensity recorded for that frequency increased to account for optical losses is subtracted by inversion in amplifier and inverter 106 and filtering together with the signal from the unimpeded light. The recorder therefore responds to a difference between the intensity at the frequency characteristic of the element to be detected and that at a frequency set by the setting of the axis of rotation of disc 110. It is, therefore, representative of the radiation due to the element corrected for a change in background radiation during passage of the sample.

We claim:

1. Apparatus for determining the elemental analysis of a carbon-containing sample which comprises: means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the detected intensity for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which the means for correcting the intensity detected for individual elements other than carbon comprises means for deducting a value related to a carbon analysis from a value detected for that element.

2. Apparatus as claimed in claim 1 which comprises means to apply a different correction at the same level of carbon analysis to frequencies characteristic of individual elements of widely separated frequencies.

3. Apparatus for determining the elemental analysis of a carbon-containing sample which comprises means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the intensity detected for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which said means for resolving radiation emitted by the plasma according to its frequency comprises a single channel spectrometer and in which the spectrometer includes means for securing a frequency shift in its setting which comprises means optically to re-set the spectrometer by a pre-set amount during a period in which the intensity of radiation of the frequency characteristic of an individual element for which it is set is detected, and in which the means for securing the frequency shift comprises a sheet-like component of a material which is transparent and refractive to the radiation and means to interpose it to transversely intersect the plane of radiation of the frequency characteristic of the element to be analyzed by rotation, the axis of rotation of the component being adjustable so as to vary the angle between the plane of the radiation and the plane of rotation by a desired amount.

4. Apparatus for determining the elemental analysis of a carbon-containing sample which comprises means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the intensity detected for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which said means for resolving radiation emitted by the plasma according to its frequency comprises a multi-chhannel spectrometer which comprises means to sense radiation characteristic both of carbon and of other elements and means for deducting a proportion of a signal representative of a carbon analysis from signals representative of the analysis of another element.

5. A multi-channel spectrometer as claimed in claim 4 which comprises means for deducting different proportions of a signal representative of a carbon analysis from signals representative of the analysis of at least two other elements.

6. A method of determining an elemental analysis of a carbon-containing sample which comprises dissociating a sample to atoms in a plasma, resolving atomic radiation emitted by the plasma according to its frequency, detecting the intensity of radiation of frequencies characteristic of carbon and of one or more other elements and reducing the intensity detected for radiation of frequencies characteristic of each of the other elements by a proportion of the intensity detected at a frequency characteristic of carbon. 

1. Apparatus for determining the elemental analysis of a carboncontaining sample which comprises: means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the detected intensity for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which the means for correcting the intensity detected for individual elements other than carbon comprises means for deducting a value related to a carbon analysis from a value detected for that element.
 2. Apparatus as claimed in claim 1 which comprises means to apply a different correction at the same level of carbon analysis to frequencies characteristic of individual elements of widely separated frequencies.
 3. Apparatus for determining the elemental analysis of a carbon-containing sample which comprises means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the intensity detected for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which said means for resolving radiation emitted by the plasma according to its frequency comprises a single channel spectrometer and in which the spectrometer includes means for securing a frequency shift in its setting which comprises means optically to re-set the spectrometer by a pre-set amount during a period in which the intensity of radiation of the frequency characteristic of an individual element for which it is set is detected, and in which the means for securing the frequency shift comprises a sheet-like component of a material which is transparent and refractive to the radiation and means to interpose it to transversely intersect the plane of radiation of the frequency characteristic of the element to be analyzed by rotation, the axis of rotation of the component being adjustable so as to vary the angle between the plane of the radiation and the plane of rotation by a desired amount.
 4. Apparatus for determining the elemental analysis of a carbon-containing sample which comprises means for exposing the sample to a plasma, means for resolving radiation emitted by the plasma according to its frequency, means for detecting the intensity of radiation of frequencies characteristic of one or more individual elements and means for correcting the intensity detected for radiation of frequencies characteristic of one or more individual elements other than carbon to account for an increase in background radiation of that frequency due to carbon, and in which said means for resolving radiation emitted by the plasma according to its frequency comprises a multi-chhannel spectrometer which comprises means to sense radiation characteristic both of carbon and of other elements and means for deducting a proportion of a signal representative of a carbon analysis from signals representative of the analysis of another element.
 5. A multi-channel spectrometer as claimed in claim 4 which comprises means for deducting different proportions of a signal representative of a carbon analysis from signals representative of the analysis of at least two other elements.
 6. A method of determining an elemental analysis of a carbon-containing sample which comprises dissociating a sample to atoms in a plasma, resolving atomic radiation emitted by the plasma according to its frequency, detecting the intensity of radiation of frequencies characteristic of carbon and of one or more other elements and reducing the intensity detected for radiation of frequencies characteristic of each of the other elements by a proportion of the intensity detected at a frequency characteristic of carbon. 